What is Kelvin-Helmholtz contraction? Why is it ruled out as a source of the present-day Sun's energy?

The Sun, a massive celestial body at the heart of our solar system, is an extraordinary source of energy. Its power helps sustain life on Earth, drives weather systems, and influences the very nature of our planet's environment. This vast energy comes from nuclear fusion, particularly from the fusion of hydrogen atoms into helium in the Sun's core.

Within the core's extreme temperatures and pressures, hydrogen nuclei overcome their natural repulsion and fuse together. This fusion process releases an enormous amount of energy in the form of light and heat, which we perceive as sunlight. Far more long-lived and stable than any contraction-based mechanism, nuclear fusion is the true powerhouse of the Sun, allowing it to shine consistently for approximately 10 billion years over its lifetime.

Gravitational contraction refers to the process where a celestial body, such as a star, decreases in volume under the influence of its own gravity, leading to an increase in its internal temperature. This phenomenon played a more prominent role during the early stages of a star's life, when it was still gathering material and settling into equilibrium.

For stars like our Sun, this contraction provided an initial heat source, as gravitational potential energy got transformed into thermal energy. However, this method of heating is not sustainable, as it relies on the continuous contraction of the star. Therefore, gravitational contraction doesn't serve as a long-term energy solution compared to the billions of years of energy produced by nuclear fusion.

Stellar evolution is the process by which a star changes over the course of time, governed by the delicate balance between gravitational forces and the pressure from internal heat. Starting with their birth from the gravitational collapse of dense clouds of gas and dust, stars undergo various stages of life. The main sequence, where our Sun currently resides, is the stage in which a star spends the majority of its life.

The star's future will depend on its mass, dictating whether it will swell into a red giant, explode as a supernova, or quietly shrink into a white dwarf. Across all stages, nuclear reactions in the star's core play a critical role, with different elements undergoing fusion as the star ages and its internal conditions change.

At the heart of stellar luminosity lies the powerful process of nuclear fusion. For medium-sized stars like our Sun, fusion occurs when hydrogen nuclei fuse to form helium, releasing vast amounts of energy in line with Einstein's famous equation, E=mc^2.

In much heavier stars, nuclear fusion progresses through various stages, creating heavier elements like carbon, oxygen, and iron. The conditions required for nuclear fusion—extreme temperature and pressure—are found in the cores of stars. These nuclear reactions not only light up the stars but also play a significant role in the chemical evolution of the Universe by producing the various elements that make up galaxies, planets, and eventually, life itself.